Zirconium is widely used as a structural material in the nuclear industry, because of its corrosion resistance, good mechanical properties, and low neutron absorption cross-section. In order to use zirconium for nuclear- grade applications, impurity concentrations of hafnium must first be removed. Hafnium inevitably occurs naturally with zirconium ores, having very similar chemical properties; hafnium, however, has a much higher neutron absorption cross-section. Removal to 100 ppm or less is therefore required.
Because of the chemical similarity of these two elements, their separation is difficult, likened to that of isotopes of the same element. A large number of technologies have been studied to accomplish this separation, with aqueous-organic solvent extraction being the procedure used in the United States. This process is effective at removal not only of hafnium, but also a number of other contaminant metals which occur in the zircon sand used as feed, including iron, aluminum, and titanium. Solvent extraction, however, is a complex process which is difficult to control, requires multiple processing steps (including aqueous solvation of ZrCl.sub.4 and subsequent chlorination back to the non-aqueous tetrachloride), and generates large quantities of waste solutions. Because of the cost and potential environmental liability associated with this process, the industry has long expressed interest in non- aqueous processing, in which the Zr-Hf separation would be carried out in the non-aqueous chloride phase.
The technique currently in use in the United States involves liquid-liquid extraction of aqueous zirconyl chloride thiocyanate complex solution using methyl isobutyl ketone, generally as described in U.S. Pat. No. 2,938,679, issued to Overholser on May 31, 1960, with the removal of iron impurities prior to solvent extraction as described in U.S. Pat. No. 3,006,719, issued to Miller on Oct. 31, 1961.
Several other processes have been suggested for separation of the metal tetrachlorides generated from the ore by carbochlorination. As noted above, the use of a non-aqueous separation offers significant economic incentive over those processes requiring aqueous zirconium solutions. Direct distillation of the tetrachlorides provides one possible route, relying on the difference in boiling points between zirconium tetrachloride and hafnium tetrachloride. Unfortunately, direct distillation cannot be accomplished at near atmospheric pressure, since neither tetrachloride exhibits a liquid phase except at very high pressure. U.S. Pat. No. 2,852,446, issued to Bromberg on Sept. 16, 1958, describes a high pressure distillation process where the pressure, rather than a solvent, provides for a liquid phase.
U.S. Pat. No. 2,816,814 issued to Plucknett on Dec. 17, 1957, describes extractive distillation for separation of the tetrachlorides using a stannous chloride solvent. U.S. Pat. No. 2,928,722 to Scheller, issued Mar. 15, 1960, describes the batch fractional distillation of niobium and tantalum chlorides to separate these chlorides from each other and from other chloride impurities, and uses a "flux" to provide the molten salt phase, utilizing either zirconium tetrachloride-phosphorus oxychloride complex or an alkali metal chloride and aluminum (or iron, or zirconium) chloride mixture as the flux. U.S. Pat. No. 3,966,458 issued to Spink on June 29, 1976 provides a sodium-potassium-zirconium chloride solvent for use in the extractive distillation of zirconium and hafnium tetrachlorides. U.S. Pat. No 3,671,186 issued to Ishizuka on June 20, 1972 utilizes a series of reaction and dissociation stages with a solvent such as sodium chloride. U.S. Pat. No. 4,021,531 issued to Besson on Apr. 3, 1977, utilizes extractive distillation with an alkali metal chloride and aluminum (or iron) chloride mixture as the solvent. Extractive distillation of zirconium (hafnium) tetrachloride with a pure zinc chloride solvent has been attempted (Plucknett et al., AEC Report ISC-51, 1949), but was unsuccessful due to the formation of a highly viscous two-phase system. The anomalously high viscosity of zinc chloride is described by MacKenzie and Murphy (J. Chem. Phys., 33, 366, 1960). U.S. Pat. No. 4,737,244 to McLaughlin et al. describes an extractive distillation method for separating hafnium from zirconium of the type wherein a mixture of zirconium and hafnium tetrachlorides is introduced into a distillation column, with a molten salt solvent being circulated through the column to provide a liquid phase, and the improvement comprising having a molten salt solvent composition of at least 30 mole percent zinc chloride and at least 10 mole percent of lead chloride. A somewhat similar process for zirconium-hafnium separation is described in U.S. Pat. No. 4,749,448 issued June 7, 1988 to Stoltz et al, which provides for zirconium-hafnium separation by extractive distillation with the molten solvent containing at least 80 mole percent zinc chloride, with the remainder including a viscosity reducer of magnesium chloride, calcium chloride, or mixtures thereof.
Of all the molten salt distillation processes, only the above-mentioned Besson process has been brought to commercial development. This process is currently in use by Cezus in France and provides product zirconium tetrachloride, relatively depleted of hafnium tetrachloride in the liquid bottoms stream, and a hafnium tetrachloride enriched vapor stream taken from the top of the column. A relatively high reflux is provided by a condenser at the top of the column and a reboiler at the bottom of the column. Because of the stability of the double salts formed with the alkali metal chloride in the solvent, it is very difficult to completely separate the product zirconium tetrachloride from the solvent, and relatively high (e.g. 500.degree. C.) temperatures are required. Aluminum chloride in excess of 1:1 molar to alkali metal chloride is required and there is considerable carry-over of aluminum chloride into the zirconium tetrachloride leaving the stripper. French Patent No. 2,543,162 (9-28-84) to Brun and Guerin describes an essentially constant temperature post-stripping process for removing aluminum chloride from zirconium tetrachloride. In addition, it should be noted that aluminum chloride is an especially hygroscopic and corrosive molten salt, and, at higher temperatures, is very difficult to handle.
One problem which may exist in such systems is the necessity of separate processing steps to remove metallic impurities. In the POCl.sub.3 complex separation process, for example, iron will concentrate in the bottoms ZrCl.sub.4 stream, titanium in the tops HfCl.sub.4 stream, while aluminum will partition between the two. If the feed contains unacceptable levels of iron or aluminum (based on the requirements for the final zirconium product), additional processing must be incorporated to remove these contaminants before synthesis of the POCl.sub.3 complex.
In addition, iron and aluminum are generally undesirable impurities in hafnium metal as well. Aluminum is especially difficult to remove down to acceptable levels in commercial aqueous processing. Further, iron is especially undesirable in the oxides of zirconium and hafnium, as the iron is not generally removed in the less extensive processing of the ore for the making of ceramics (typically zirconium-2% hafnium oxide), leading to undesirable discoloration of the ceramic.